GPS was developed as a ranging system to determine unknown positions on land, sea, in air and space based on the known positions of a constellation of 24 satellites in 6 orbital planes. The orbital planes, each having 4 satellites positioned therein, are inclined at 55 degrees with their ascending nodes each being separated by 60 degrees. In this regard, each one of the GPS satellites travels in a semi-synchronous circular orbit around the air space above the surface of the earth. The orbits traveled by each satellite are broadcast and superimposed on a set of GPS pseudorandom noise codes and are available to obtain precise ephemerides after post-processing. A conventional GPS receiver, tuned to receive the GPS pseudorandom noise codes, converts the codes into time, position and velocity estimates that are sufficiently accurate for navigation, positioning and time dissemination.
Each one of the GPS satellites transmits its pseudorandom noise codes on two basic frequencies using on an board atomic clock that produces a fundamental L-band frequency of 10.23 MHz. Using carrier frequencies generated by multiplying the fundamental frequency by 154 and 120 respectively, the two basic frequencies of 1575.42 MHz and 1227.60 MHz are formed to carry two pseudorandom noise codes. The two pseudorandom noise codes are superimposed on the basic carrier frequencies along with other satellite emphemerides such as status information, system time, and satellite clock corrections. The measured travel time of the signals traveling from each satellite is then utilized by a GPS receiver to compute pseudoranges and time. In summary then, the use of the Global Positioning System to provide position and precise timing information is well known in the prior art.
While the use of GPS to provide position and precise timing information is well known in the prior art such GPS receivers for most part consume large amounts of energy and thus, have found limited portable use.
One attempt at solving the problems associated with power consumption is disclosed in U.S. Pat. No. 5,883,594 by Chung Y. Lau, entitled "GPS Receiver Using a Message System for Reducing Power Consumption." The Lau patent describes a GPS message system including a GPS base station for receiving GPS signals and providing GPS acquisition and location information and a message system manager for transmitting a radio message signal including a wakeup call and GPS information. In this regard, a message transceiver receives the radio message signal and passes the wakeup call and GPS information to the GPS receiver having a low power standby mode. The GPS receiver awakes from the standby mode and enters an operational mode for using the GPS information for acquiring the GPS signal and deriving a first location fix.
While such a system may be satisfactory for some applications, the Lau system is not entirely satisfactory as it requires large bulky secondary equipment to transmit the wakeup call and GPS information to the low power GPS receiver.
Therefore it would be highly desirable to have a new and improved low power GPS receiver system that enters into an operational mode without the use of a secondary radio transmission signal and that operates to provide precise timing information in a fast and convenient manner.
Another attempt at solving the problems associated with power consumption are disclosed in U.S. Pat. No. 5,905,460 by Odagiri, et al. entitled "Wrist Watch Type GPS Receiver." The Odagiri et al. patent discloses a GPS system that includes an antenna posture detecting device for detecting an antenna posture condition and a receiving signal operation controlling device for temporarily interrupting the GPS wave receiving operation when it is impossible for the system antenna to receive the GPS wave. The receiving signal controlling device interrupts the GPS wave receiving operation until the GPS wave receiving operation is again possible. In this manner, the operation of the GPS receiver is interrupted preventing large power losses when the GPS wave can not be received properly.
While such a system may be satisfactory for some applications, the Odagiri, et al. system is not entirely satisfactory as it requires an antenna posture detecting device as well as a moving distance calculating device for calculating a moving distance of the user to correct for that period of time when the GPS signal receiving operation is interrupted.
Therefore it would be highly desirable to have a new and improved low power GPS receiver system that operates to provide precise time information without interrupting normal operations and without the use of special antenna posture detecting or moving distance calculating devices.